The intake manifold temp sensor triggers the fans to run at a noticeably slower speed only when the intake manifold is quite hot and only when the ignition is off. A fault in that sensor or that circuit should not cause the fans to run at full speed at any time, and should not cause the fans to run when the ignition is on.
I believe that a faulty AC refrigerant switch could cause the fans to run at full speed while the ignition is on. Be sure you know which sensor on the AC receiver/dryer is the temp switch, and which is the pressure switch. It also might be possible for the radiator switch to cause this. Disconnecting the switches one at a time should show if either is the source of the problem.
Wally
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What can cause the fans to run when ignition key is turned on? Well...

What triggers fans to run when ignition key is turned off?

regards, Theo

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This is the power flow for a 1988 model

 

*

Lets think sensors:
- The AC Freon high pressure sensor at the top of the AC dryer. It is temperature-variable resistor. Porsche calls it pressure transmitter, 928.606.150.00. It looks like it combines a pressure sensor (variable resistor) and a switch (on-off type). The variable resistor informs the controller box, the "low pressure switch and max pressure switch" controls the clutch operation, and the "intermediate" pressure switch tells the controller to run full speed or half speed.

And it changed at MY 1987 it seems, things in earlier models could be different.


 

So step by step:

- The AC Freon combined pressure sensor /switch at the top of the AC dryer. 928.606.150.00

o Pressure below 2.0 bar: switch opens on low pressure. Compressor will not run when clutch is activated by the AC console.

o Pressure above 2.3 bar but below 26 bar. Compressor will run when clutch is activated by the AC console.

o Pressure above 32 bar: switch opens on high pressure. Compressor will not run even when clutch is activated by the AC console.

o Pressure sensor reads 40 ohms, selling the controller that it is low pressure. Fans will be off.

o Pressure sensor reads 60-80 Ohms. Fans should be off.

o Pressure sensor reads about 80 ohms and fans should run moderate speed ** value wrong? 60-70 ohms does not trigger fan operation yet.
o Pressure sensor reads 120 ohms and fans activate to full speed ** this might be wrong, looks like this is a switch, not a resistor
 

- The AC Freon low pressure switch at the bottom of the AC dryer. 959.613.137.00

o The switch closes at a pressure of 18 bar, and informs the controller that fans should run

o The switch opens at a pressure of 14 bar, and informs the controller that fans should not run

 

- The Coolant temperature switch at the front of the radiator, way down at the left side. This is a temp resistor 928.606.149.00. A bit awkward that Porsche talks about a temperature switch but it is actually a NTC variable resistor.

o Coolant cold: Temperature around 22 C. sensor reads 4500 ohms (or is it 3800?), fans are off.

o Coolant hot: Temperature over 79 C, sensor below 1931 ohms value, fans start to run at low speed (7,5V)
o Coolant very hot: Temperature over 85 C, sensor below 1582 ohms value, fans start to run at moderate speed (8,5V)
o Coolant extremely hot: Temperature over 95 C, sensor below 1128 ohms value, fans start to run at full speed (12V)
 

COOLANT Temperature Sensor Values

C _ R (ohms)

74 - 2300

77 - 2066

79 - 1931

85 - 1582

90 - 1330

95 - 1128

- The AC switch in the dashboard. This is the pushbutton that switches the AC on.

o Switch pressed: 12v from the Fresh air relay is sent to the controller
o Switch off: 0v is sent to the controller
 

- The intake temp switch 928.606.217.00

o Temp of the intake above 90C: switch engages and connects gnd to the controller. Fans will run moderate speed if key is removed from ignition and hood is closed.
o Temp of the intake below 90C: switch disengages and disconnects from the controller
 

- Transmission oil temp switch 928.606.152.00

o Switch engages at 110C and connects gnd to the controller
o Switch disengages below 110C and disconnects from the controller
 

- Door contact engine hood (1987 up to the 1991 model)

o Switch engaged (hood closed) enables the fans running
o Switch disengaged (hood open) disables fans from running (safety measure, fans stay off when engine is hot but not running)

Theo

1992 928 GTS midnight blue

http://928gts.jenniskens.eu